1. Field of the Invention
This invention relates to a technique for optimizing the fabrication cost and the display image quality of an EL (electro luminescence) display panel which is driven and controlled by an active matrix driving method, and more particularly to an EL display panel module, an EL display panel, an integrated circuit device, an electronic apparatus and a driving controlling method.
2. Description of the Related Art
A circuit block configuration popular to an organic EL panel module of the active matrix driven type is shown in FIG. 1. Referring to FIG. 1, the organic EL panel module 1 shown includes a pixel array section 3, and a write scan driver 5, a power supply scan driver 7 and a horizontal selector 9 which are driving circuits for the pixel array section 3.
Incidentally, an organic EL element is a current light emitting element. Therefore, the organic EL panel module adopts a driving method wherein the amount of current to flow through the organic EL element is controlled for control of the gradation of light to be emitted from each pixel. FIG. 2 shows one of comparatively simple circuit configurations for a pixel circuit of the type described. Referring to FIG. 2, the pixel circuit shown includes a scan transistor T1, a driving transistor T2 and a storage capacitor Cs.
The scan transistor T1 is used to write a signal voltage corresponding to a gradation of the corresponding pixel into the storage capacitor Cs. The driving transistor T2 is used to supply current Ids based on a gate-source voltage Vgs, which depends upon the storage voltage of the storage capacitor Cs, to an organic EL element OLED.
Incidentally, where the driving transistor T2 is formed from a p-channel type thin film transistor, it is connected at the source electrode thereof to a power supply line. In other words, the driving transistor T2 is designed so as to normally operate in a saturation region thereof. Accordingly, the driving transistor T2 operates as a constant current source. Thereupon, the current Ids is given by the following expression:Ids=k·μ·(Vgs−Vth)2/2where μ is the mobility of the majority carrier of the driving transistor T2 and Vth the threshold voltage of the driving transistor T2, and k is a coefficient given by (W/L)·Cox. Here, W is the channel width, L the channel length, and Cox the gate capacitance per unit area.
It is to be noted that, in the configuration of the pixel circuit described, the drain voltage of the driving transistor T2 varies together with the aged deterioration of the I-V characteristic of an organic EL element illustrated in FIG. 3. However, since the gate-source voltage Vgs is kept fixed, the amount of current supplied to the organic EL element does not vary, and the luminance of emitted light is kept fixed.
An organic EL display panel which adopts the active matrix driving method is disclosed, for example, in Japanese Patent Laid-Open Nos. 2003-255856, 2003-271095, 2004-133240, 2004-029791 and 2004-093682.